Mycoplasmas, the smallest free-living microorganisms, are widely distributed in nature and commonly produce disease in plants, insects, and animals, including man. Although the economic impact of these diseases is great, little information is available concerning mechanisms of pathogenesis and effective methods of control are unavailable. Recent studies have shown that the surface properties of mycoplasmas vary at a high frequency of about 10-3 per cell per generation. High-frequency phenotypic variations affect the structure of surface antigens, the morphology of mycoplasma colonies, the susceptibility of the organism to mycoplasma viruses, and the adsorption of mycoplasmas to red blood cells. The assumption is that high-frequency changes in mycoplasmal surface properties are also important to disease pathogenesis. We have recently found that the chromosome of mycoplasma pulmonis undergoes recombinative events at a frequency comparable to phenotypic variation. We have also shown that a repetitive element is present in the chromosome of Mycoplasma pulmonis and may be associated with genetic recombination. Long-range goals are to understand the molecular basis of mycoplasmal chromosomal and phenotypic variation, the role these phenomena play in disease pathogenesis, and the role they have played in mycoplasmal evolution. One aim of this proposal is to examine the mechanism(s) of genetic recombination in mycoplasmas. This will be accomplished by DNA sequence analysis of the precursors and products of recombinative events. Another aim is to measure the frequency of chromosomal rearrangements in general and to determine whether specific recombinative events correlate with phenotypic variation. In related studies, we have recently shown that the recE gene from Bacillus subtilis hybridizes to DNA from another mycoplasma, Acholeplasma laidlawii. Therefore, a third aim of this proposal is to use the recE gene (analogue to the recA gene of Escherichia coli) as a hybridization probe to isolate the related A. laidlawii sequences in order to study the mycoplasmal analogue of this important gene.